Notably in chemical and oil industries the metal pipes to be buried in the ground have to be protected against corrosion. This is done by applying a synthetic resin layer thereto. The known method is disclosed in the German patent application Offenlegungsschrift No. 2 257 135 and utilizes, as does this application and the invention thereof, inductive heating of the metal pipe or object to induce flow of the thermosetting resin and curing thereof. In this known method, however, the inductive heating is carried out with a sufficiently high frequency of energization of the inductive coil as to effect a differential heating of the metal pipe, i.e., to produce the highest temperature in the skin thereof and lesser temperatures inwardly of the skin. Moreover, in the prior art in general, the pipe if of large dimensions is subjected to preheating to an intermediate temperature prior to wrapping the pipe, the preheated temperature typically being in the order of about 80.degree. C. To this end radiation heat may be used instead of the inductive heating mentioned above. In any event, the objective of the preheating is so that as soon as the coating material with the resin comes into contact with the pipe the resin starts flowing, by which a satisfactory seal is intended to be obtained between the various turns of the wrapping material or tape.
The invention has for its object to improve the known method in a manner such that the seal is better ensured and energy can be saved.
According to the invention these two objectives are achieved in that during the winding operation the pipe is maintained approximately at room temperature and subsequent to the winding operation the pipe is gradually and subsequently uniformly heated throughout its thickness progressively from one end to the other end thereof up to a first temperature, at which the resin becomes fluid and is maintained at said first temperature while the pipe is rotated about its longitudinal axis for a time sufficient to allow the resin to spread and assure that the ultimate sealing and insulation is most effective. In addition, the frequency of the alternating current which energizes the induction coil during heating is chosen such that the whole object is gradually heated, i.e., it is not differentially heated as in the prior art but is substantially uniformly heated throughout its thickness. Since the pipe is not preheated, the resin hardly fluidizes upon winding. Subsequent to completion of the winding operation and the gradual and uniform heating of the pipe to the first temperature, the resin will slowly and quite gradually become fluid and hence a satisfactory spread and a satisfactory seal and insulation are ultimately obtained. Moreover, the required amount of thermal energy is lower. The first temperature range lies around about 90.degree. C. This first temperature range can be stepwise attained. Some time after the first temperature range is reached the pipe is further heated to about 132.degree. C., a temperature sufficient to cure the thermosetting resin and subequent to curing the pipe is allowed to cool down. Thus, the heating is effected not only subsequent to the wrapping but also in a plurality of stages from room temperature to the final temperature at which curing of the resin takes place. In the process, a resin-flowing temperature is first attained and then a resin-curing temperature is attained. Heating of the pipe is carried out with the aid of an induction device in which the pipe is passed through an induction coil which is fed by an alternating voltage of a frequency lying in the medium frequency range so as to effect the aforesaid uniform heating. Thus the whole pipe is progressively heated. When feeding by an alternating current frequency in the high frequency range as proposed in the prior art described only the outer zone of the pipe is heated due to skin effect. When heating in the medium frequency range as in this invention, the thermal capacity becomes higher.